The fourth secret ending is unlocked by following cryptic directions found on a stone tablet in a secret room inside of a very deep cave located under the nailmaster in the edge of the kingdom, which unlocks after 100%ing the game or getting the third ending (not sure which).

To my knowledge this is the first time this ending has been completed.

This was an incredibly fun journey, and while I still have some mysteries to solve I will leave those to others as I am going on holiday for a week.